Three phase Induction motors form the heart of Industrial systems. Thus healthy motors contribute to a healthy industry and high industrial growth. It has been observed that one of the prominent reasons for motor failure is unregulated voltage at the user end. Both low and high voltages affect the motor performance and motor deterioration. Let us have a look at these effects to have a better understanding of motor performance.
The graph below shows the effect of voltage sag or swell on motor performance.
Low Voltage:
As seen from the above graph, as input voltage dips, it results in rise of Full Load Current as Shaft Power drawn will remain constant. This will result in increase of Copper/Winding Losses and overheating. Most of the time motors can withstand under voltages due to the tune of 10% but any sag below it can result in dangerously high losses. Thus it reduces efficiency of the motor.
Motor torque drops with drop in voltage as it is proportional to square of voltage. Thus for 90% voltage, torque will be reduced to 81%. In case of heavy loads, this may result in locked rotor and winding overheating. Thus, care must be taken in case of loads with high starting torque. In case of light loads like pumps, reduced voltage will reduce mechanical stresses and result in better starting. In fact, reduced voltage starting is one of the recommended methods for starting of light loads.
As magnetizing current is low in case of low voltage, motor will have improved PF .
High Voltage :
As input voltage rises, Full load current is expected to dip. However, higher voltages result in higher magnetizing currents and higher Iron/Core Losses. This in fact results in higher full load currents and insulation overheating. Most of the time motors can withstand overvoltage due to the tune of 10% but any rise above it can result in dangerously high losses. Thus it reduces efficiency of the motor.
Motor torque rises with rise in voltage as it is proportional to square of voltage. Thus for 110% voltage, torque will rise to 121% . It will also result in rise of starting current resulting in overheating and very high mechanical stresses . This might damage the motor and/or the load. Hence, overvoltage is very dangerous for a motor.
As magnetizing current is rise , motor has lower PF in case of overvoltage .
Thus we can conclude that both overvoltage and under voltage are dangerous for motor health. Hence, a regulated supply is necessary to achieve best performance. As we cannot rely on utility to regulate the supply, it is recommended to use a On Load Tap Changer with the Transformer to achieve Voltage regulation.
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